Because of the rapid development of network and multimedia technologies, various multimedia data such as video files can be easily accessed through network. Although convenience of using the accessed multimedia data is undeniable, there are potential risks for security of the multimedia data. For example, pirates can spread huge multimedia data at low costs without authorization from creators or owners of the multimedia data, and such piracy impacts copyright and discourages creations. It is necessary to combine copyright information to the multimedia data to inhibit copyright infringement. A known method is to embed digital watermark in the multimedia data. Digital watermarking is a process of hiding digital information, which may include copyright information, in redundant data of the multimedia data according to a specific algorithm. Digital watermarks may be used to verify the authorization or integrity of the multimedia data so as to protect the intellectual property of legal owners of the multimedia data.
The digital watermark is usually embedded in the multimedia data before distribution or during playing. However, such action cannot effectively reduce piracy because the invisible digital watermark can be eliminated by non-contact image-capturing or recording. Even though an illegal copy is seized, the legal owner cannot trace the original distributor of the illegal copy since the digital watermark is removed and cannot be restored for inspection.